The present invention relates, in general, to a system, apparatus and method for polymerizing propylene including a propylene pressurization vessel operated under supercritical conditions.
Without limiting the scope of the present invention, the background of the invention is described with reference to propylene polymerization and copolymerization reactors and systems. Conventional propylene polymerization systems have been operated with temperature and pressure ranges well below the critical region of propylene, for example, below a critical temperature (Tc) of 197.2xc2x0 F. and a critical pressure of 655.4 psig. For example, in more than one conventional process the polymerization system is typically operated at a temperature of about 140xc2x0 F. and 180xc2x0 F. and a pressure in the range of 440 to 480 psig. Surprisingly, however, it has been discovered that with presently available propylene polymerization catalysts, production is increased if the systems are operated outside of the conventional temperature and pressure ranges previously employed.
However, in order to take advantage of the unexpected benefits of operating outside of conventional temperature and pressure ranges, it has been found necessary to provide a propylene pressurization vessel that is operated above the critical temperature and pressure range of propylene, e.g., in the supercritical state. Thus, there exists a need for a propylene polymerization system that includes a pressurization vessel adapted to operate outside of conventional parameters.
The present invention provides a method and apparatus for pressurizing a propylene polymerization system, including a pressurization vessel operated under supercritical conditions. The vessel contains propylene in a first region at supercritical conditions and propylene in a second region at subcritical conditions. In one embodiment of the invention, pressure in the pressurization vessel is controlled by the rate of injection of superheated propylene in the supercritical region. The temperature of propylene in the zone containing the compressible supercritical phase may be maintained by controlling the temperature of the superheated propylene feed. Preferably the temperature of the superheated propylene is in the range of 200xc2x0 F. to 280xc2x0 F., more preferably, the temperature of the superheated propylene is approximately 240xc2x0 F. The pressure of the vessel is preferably in the range of 660 to 800 psig, more preferably in the range of 700 to 730 psig, most preferably in the range of approximately 710 psig. The temperature of the dense subcooled liquid propylene in the vessel is typically in the range of 80xc2x0 F. to 140xc2x0 F., preferably 90xc2x0 F. to 110xc2x0 F., and most preferably approximately 100xc2x0 F. Operation of the pressurization vessel in this mode allows a propylene polymerization system to be pressurized and operated at levels above conventional polymerization temperatures and pressures, for example, at pressures of 660 to 700 psig.